Lifetime
One Lifetime, Generation, One Generation
This word has the kanji for one and period. You only have one period to live in so this is the only lifetime you get. Unless you're reborn, but that will probably be in the next age, not this one. So one lifetime for you!
いちだい
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
かれは、一代でここまでかいしゃを大きくした。
He made the company this much larger in just one generation.
コウイチは一世一代の大しょうぶに打って出た。
Koichi went to take a once-in-a-lifetime gamble.
彼は一代で巨額の財産を作ったが、彼の息子がさらにそれを二倍にした。
He amassed an enormous fortune over the course of his lifetime and his son doubled it.